156 MR JAMES MURRAY ON 
S. tremula, are common in our small lakes; some of the other species may atfect 
more particularly larger lakes; but no one species is general in the lakes, and it is 
not by any means the case that any Synchzta is invariably present. Many lakes have 
normally no Syncheta. 1 am inclined to regard all the Syncheetadze, like all the 
Ploesomadee, as local species. 
Triarthra longiseta is more difficult to deal with. It looks a thoroughly limnetic 
animal ; it has a wide distribution in Scotland ; and, being more frequently seen in winter 
and early spring, it may have been overlooked in some lochs, and may be commoner 
than we know. Still, the fact remains that we have only seen it in some twenty-four 
lochs, and of these only five are moderately deep, while it is absent from all our greatest 
lochs. It is less common than Gastropus stylifer and Floscularia pelagica, which are 
considered local species. While | am not prepared to trace the universal distribution 
of the species in lakes, I would point out two facts which confirm our experience of it. 
JENNINGS (26) does not indicate that it is one of the common limnetic species in 
the Great Lakes, giving only one record, from Sandusky Bay. Zacwarras (56) 
describes a var. /imnetica, implying that the type is not limnetic; but the variety 
appears to be rare. The species is found in the plankton lists of many European 
biologists, but it must be remembered that most of the biological stations are established 
on shallow lakes. 
As to Anurea aculeata, our experience runs quite counter to Dr Lunp’s. The 
species has not, to my knowledge, ever occurred in a purely limnetic collection from 
any lake in Scotland. The type of the species is rare even in littoral collections. 
Several varieties—A. serrulata, A. brevispina. and A. valga—are of more frequent occur- 
rence among weeds. Of these 4. valga is most nearly limnetic, being abundant in the 
plankton of a number of small and shallow lakes; but it also is absent from the larger 
lakes. As in the case of Tivarthra longiseta, JENNINGS’ (26) few records indicate that 
it is not a common lacustrine species in America. As to its presence in the plankton of 
many European lakes, the same remarks apply as to Trvarthra. 
Besides the five cosmopolitan plankton Rotifers. there have been observed in the 
Scottish lochs a number of other species, as thoroughly limnetic, but of more local 
distribution. These are Floscularia pelagica, Floscularia mutabilis, Triarthra 
longiseta, Polyarthra euryptera, Syncheta pectinata, Syncheta tremula, Gastropus 
stylifer, Plasoma hudson, Plaesoma truncatum, Anapus testudo, and Conochilus 
volvox. 
Proales (Hertwigia) parasite, though not itself limnetic, is carried with its host 
(Volvox) into the open water of many lakes, and some even of our great lakes. 
Gastropus stylifer is the commonest of these species. It has been found in about 
seventy lochs distributed over the whole of the mainland and islands. 
Conochilus volvow may be as common, or even more common; but, as it is not so 
easily recognisable when dead and contracted, we have fewer records for it. It is 
widely distributed. 
